Low temperature material bonding technique

ABSTRACT

A method of performing a lower temperature bonding technique to bond together two mating pieces of glass includes applying a sodium silicate aqueous solution between the two pieces.

This application is a division of Ser. No. 08/645,497 filed May 16,1996, now abandoned.

This invention was made with Government support under contractDE-AC05-840R21400 to Lockheed Martin Energy Systems, Inc. and theGovernment has certain rights in this invention.

FIELD OF THE INVENTION

The present invention relates to a method of bonding glass components attemperatures below 100° C., and more specifically, to a method ofbonding two mating pieces of material together using a monomolecular orquasi-molecular film that covalently bonds to both surfaces.

BACKGROUND OF THE INVENTION

Low temperature bonding methods can enable technology by allowing themanufacture of devices that include a broader range of materials.Temperature processing can limit materials selection due todecomposition, vaporization, dissolution, or coefficient of expansion.For example, a variety of micro devices for chemical and biochemicalanalysis are being developed.

The fabrication of many of these devices requires the bonding of glasscomponents, such as the bonding of a glass cover plate to a photolithographically etched glass substrate, in order to produce a devicecontaining closed micro channels. This bonding process has previouslybeen carried out by fusing the two components at high temperature (e.g.500-1,100° C.), preventing the inclusion of bio-molecules or othertemperature-sensitive materials in open channels. It also allows the twocomponents to have different thermal coefficients of expansion.

Currently under development are devices for the analysis of nucleicacids. These devices require the attach;ment of nucleic acid probes tospecific sites within the micro channels formed in a glass substrate,which is most efficiently carried out in open channels prior to bondinga cover plate to the substrate. However, conventional, high temperaturebonding techniques would damage the probes and thus compromise theintegrity of the analysis. Moreover, adhesive bonding techniques areundesirable given the very small (micron order) dimensions of the microchannels.

Thus, a continuing need exists for a low temperature bonding technique.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a low temperaturebonding process that will allow the incorporation of DNA probes or othertemperature-sensitive molecules on the substrates and facilitatemanufacture of such devices.

Another object of the present invention is to provide a method ofbonding two different types of glass substrate materials together or theinclusion of thin metal films on a structure prior to bondings. In theformer case, high temperatures preclude bonding due to differentcoefficients of thermal expansion. After bonding at high temperature,stress is introduced by the variation in dimension upon cooling to roomtemperature.

Another object of the present invention is to provide a low temperaturebonding technique that obviates problems associated wiht metal filmswhich are deposited at high temperatures, by avoiding vaporization ordiffusion into the substrate material.

These and other objects are achieved by providing a method of bondingtwo mating pieces of material together which includes applying anaqueous solution of sodium silicate to a surface of one of the twomating pieces, and then contacting the mating pieces with the aqueoussolution therebetween.

Preferably, the solution is spun-on at high speed on the one piece, andthen the other piece is immediately brought into contact.

Other objects, advantages, and salient features of the invention willbecome apparent from the following detailed description, which taken inwith the annexed drawings, discloses the preferred embodiment of thepresent invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a mechanism of dissolution of silica in water in thepresence of hydroxyl ions;

FIG. 2 illustrates a deposition mechanism for silica on a solid surface;

FIG. 3 illustrates the bond formation between silica particles;

FIG. 4 illustrates bonding between silica particles through coordinationwith flocculating metal cations;

FIG. 5 is a graph illustrating electro-osmotic flow of a chip made fromsodium silicate bonding (7 wt. % and 90° C. for 1 hour);

FIG. 6 is a graph illustrating electro-osmotic flow of a chip made fromdirect bonding;

FIG. 7 is a graph illustrating the effect of sodium silicate solution inthe channel on the electro-osmotic flow;

FIG. 8 is an example of a microchip formed according to the methodologyof the present invention;

FIG. 9 illustrates a method of fabricating an in-column electrode usingtwo masks;

FIG. 10 illustrates the sequence of steps in forming the electrode;

FIG. 11 is an enlarged top view of the cross area of a microchip andelectrode formation; and

FIG. 12 is a top view of a microchip and electrode formation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Silica-water systems are discussed thoroughly in The Chemistry of Silicaby R. K. Iler (Wiley, New York 1979), wherein it was noted that as wateris a unique liquid, so is amorphous silica a unique solid. They aresimilar, both consisting mainly of oxygen atoms with the smallerhydrogen or silicon atoms in the interstices. There is no evidence thatsilica is more soluble in any other liquid than water through hydrolysisforming silicic acid Si(OH)₄:

SiO₂+2H₂O⇄Si(OH)₄

Supersaturated solutions of silicic acid in pure water arethermodynamically unstable because condensation polymerization throughdehydration takes place. Thus, the dissolution and deposition of silicain water involves hydration and dehydration reactions catalyzed by OH⁻ions:

2SiO₂+2H₂O⇄SiO₂+Si(OH)₄

When the solution is highly supersaturated and insufficient solid silicasurface is available to permit rapid deposition of soluble silica, newsmall nuclei particles are formed by intercondensation on monomer andlow polymers. Silica is also deposited on these nuclei untilsupersaturation is relieved. It is in this manner that colloidalparticles of silica are formed. See The Chemistry of Silica, supra, pp.3-6.

The reported solubility values for amorphous silicas range from 70 tomore than 150 ppm SiO₂ at 25° C. due to differences in particle size,state of internal hydration, and the presence of traces of impurities.There is an apparent increase in the solubility from pH 9 to 10.7, owingto the formation of silicate ion in addition to the monomer Si(OH)₄which is in equilibrium with the silica solid phase:

2SiO₂+2H₂O⇄SiO₂+Si(OH)₄

Si(OH)₄+OH⁻═Si(OH)₅ ⁻ (or HSiO₃ ⁻+2H₂O)

Above pH 10.7, all the solid phase of amorphous silica dissolves to formsoluble silicate, since at higher pH the concentration of Si(OH)₄ isgreatly lowered by conversion to ionic species, so that no amorphoussolid can remain in equilibrium. The sodium silicate is usually writtenas SiO₂.Na₂O which is from:

HSiO₃ ⁻+OH⁻═SiO₃ ²⁻+H₂O

Na₂SiO₃═SiO₂+Na₂O

The dissolution process of silica requires the presence of a catalystwhich can be chemisorbed and increases the coordination number of asilicon atom on the surface to more than four, thus weakening the oxygenbonds to the underlying silicon atoms. The hydroxyl ion is the uniquecatalyst in alkaline solutions and fluoride ion in hydrofluoric acidsolutions.

FIG. 1 shows a mechanism of dissolution of silica in water in thepresence of hydroxyl ions. The first step is the adsorption of OH⁻,after which a silicon atom goes into solution as a silicate ion. If thepH is much below 11, the silicate ion hydrolyzes to Si(OH)₄ and OH⁻ andthe process is repeated. Hydrofluoric acid probably acts in the sameway, the first step being chemisorption of a F⁻ ion, which is about thesame size as OH⁻ ion. Above pH 11 the hydroxyl ions keep convertingSi(OH)₄ to silicate ions, thus keeping the solution unsaturated so thatsilica continues to dissolve.

Silica can be deposited molecularly from supersaturated aqueoussolution. The mechanism for deposition of SiO₂ from Si(OH)₄ isapparently the reverse of dissolution of solid silica. Supersaturationcan be brought about by lowering the pH of an aqueous solution of asoluble silicate. The deposition involves a condensation reactioncatalyzed by hydroxyl ions. Therefore, the process occurs principallyabove pH 7 where the hydrolysis of silicate ions occurs, but not abovepH 11 where silica dissolves as silicate ion. The degrees ofsupersaturation must be sufficient for deposition to occur, but must notbe so great as to allow the formation of colloidal particles, or theprocess becomes very inefficient.

FIG. 2 shows the silica deposition on a solid surface. Monomeric silicaSi(OH)₄ condenses with any preexisting solid surface that bears OHgroups with which it can react, namely, SiOH, or any MOH surface, whereM is a metal that will form a silicate at the pH involved. Once areceptive surface is covered by the reaction shown in FIG. 2, thefurther deposition is silica on silica, thus building up a film.

There is no analogy between silicic acid polymerized in an aqueoussystem and condensation-type organic monomers which polymerize intolinear chains and then are branched and cross-linked. The polymerizationof silica is recognized in three stages: (1) polymerization of monomerto form particles; (2) growth of particles; and (3) linking particlestogether (aggregation) into branched chains, then networks, finallyextending throughout the liquid medium, thickening it to a gel.

Thus, formation and growth of spherical particles is one kind ofpolymerization and aggregation of particles to form viscous sols andgels is another kind of polymerization. Both types of polymerization mayoccur at once and have the same mechanism of condensation to formsiloxane (Si—O—Si) bonds. The polymerization of monomer to formparticles occurs once the monomer is at a concentration greater than thesolubility of the solid phase of amorphous silica. At low pH the silicaparticles bear very little ionic charge and thus can collide andaggregate into chains and then gel networks. Above pH 6 or 7, and up to10.5, where silica begins to dissolve as silicate, the silica particlesare negatively charged and repel each other. Therefore, they do notcollide, so that particle growth continues without aggregation. It is inthis way that the more alkaline sols are stabilized. Sodium silicatesolution is quite stable at pH 12, but the pH may drop in storage owingto absorption of atmospheric carbon dioxide.

FIG. 3 shows the mechanism of gel formation through inter-particlebonding to form Si—O—Si bonds when particles collide. Iler also proposeda mechanism for particle coagulation which involves cationic bridgingagents. FIG. 4 shows the series of events which is postulated to occuras silica particles are bridged by Na⁺ cations. Firstly, surfacehydroxyls transfer protons to the hydrogen-bonded water layer; next, asodium ion is adsorbed at the negative site; and finally, contact withanother particle results in formation of a coordination linkage betweensilica particles.

Based on Iler's theory, Michalske and Fuller studied closure andre-propagation of healed cracks in silicate glass at relative humidities(rh) between 0.01% and 100% at room temperature. They measured the crackopening and closure forces as the strain energy release rate G whichdescribed the energy associated with increasing the length of a crackhaving unit width. Two types of glass were compared. One was vitreoussilica and the other was soda-lime-silica float glass having thecomposition of (in mol %) 75% SiO₂, 14% Na₂O, 10% CaO, 1% other. SeeClosure and Repropagation of Healed Cracks In Silicate Glass, by T. A.Michalske and E. R. Fuller, Jr., J. Am. Ceram. Soc., 68 (1985) 586.

Michalske and Fuller found that the initial bonding energy (crackclosure) was 0.15 J/m² at 50% rh for both glasses which corresponded tohydrogen bonding through bridged water molecules. The energy required toreopen a healed crack in vitreous silica glass was nearly equal to thecrack closure energy at all humidities. However, healed crack invitreous silica glass was nearly equal to the crack closure energy atall humidities. However, the energy required to reopen a healed crack insilicate glass measured 1.7 J/m² in driest conditions, much larger thanthe value that can be explained by hydrogen bonding, although athumidities higher than 70% the reopen energy was almost the same as theclosure energy. This suggested that once crack surfaces have been pulledtogether by hydrogen bonding, further reactions enhance the bridginglinkages between silicate glass surfaces.

Two mechanisms were proposed to describe the increased adhesion betweensilicate glass surfaces: one was cationic bridging and the othersiloxane bridging. Michalske and Fuller discussed that once surfaceshave been pulled together by hydrogen bonding which can connect aseparation up to 1.0 nm, the development of electrostatic bonds wouldreduce the crack separation to ≈0.4 nm (equal to the diameters of onesodium and one oxygen).

At this separation, it may be feasible to form siloxane bridges whichcompletely close the distance between crack surfaces. Since crackhealing was observed to be independent of the number of cycles, thecationic bridging mechanism was favored to explain the healing effectsat room temperature.

Using the previous work as their baseline, Michalske and Keefer laterstudied the adhesion of hydrated silicate films at differenttemperatures. See “Adhesion of Hydrated Silicate Films”, Mat. Res. Soc.Symp. Proc., by T. A. Michalske and K. D. Keefer, 121 (1987) 187. Threesilicate solutions were investigated: a sodium hydroxide stabilizedcolloidal silica (3 wt. % SiO₂), an aqueous solution containinghydrolyzed tetramethoxysilane (TMOS) (6 wt. % SiO₂), and a sodiumsilicate solution (3 wt. % SiO₂). The investigators introduced thesilicate solutions in crack surfaces of silica glass plates and thenallowed the crack to close. The samples were heat treated and the crackswere reopened at the treated interfaces. It was found that the colloidalsilicate solution showed hydrogen bonding (0.15 J/m²) up to 200° C. andno interfacial bonding at temperatures from 300 to 800° C.

It was suggested that once the water was removed, the particles did notstrongly interact with the substrate surface. They hydrolyzed TMOSsolution showed hydrogen bonding up to 400° C. At temperatures above400° C., the interfacial bond strength increased and was attributed tocondensation reactions forming siloxane bonds. The sodium silicatesolution generated the largest interfacial energy. The adhesion energymeasured after heat treatment from room temperature to 100° C. (3.7J/m²) was greater than the value predicted for simple cationic bridging(2.0 J/m²). The investigators suggested that since sodium silicatesolutions were known to polymerize at room temperature, some siloxanebond formation accompanied the cationic bond formation. With a heattreatment of 200° C. or above, an adhesion energy as large as thecohesive energy of silica glass was obtained indicating the siloxanebond formation between the silicate film and substrate surface.

From the results obtained, Michalske and Keefer concluded that filmadhesion through the formation of siloxane bonds to the silica substrateis most likely to occur when the solution species are polymeric innature; i.e., the silicate species in solution are not highlycrosslinked and have high reactivity. In the case of sodium silicatesolution, there is also the effect of cationic bridging that must beconsidered in the measured adhesion.

Quenzer and Benecke used thin sodium silicate layers to bond twooxidized silicon wafers together and the method was a modified processfor silicon direct bonding to reduce the process temperatures. Thewet-thermal growth of SiO₂ on silicon wafer had a thickness in the rangeof 100 to 1,200 nm. After a hydrophilic treatment (hot nitric acid),diluted solutions of sodium silicate (Na₂O:SiO₂≈1:3 with concentrationsof 0.1, 0.5, 2.0, 5.0 wt. % in water) were spun (3,000 rpm) onto one ofthe two surfaces and the two wafers were brought into contactimmediately. The investigators found that after a final temperaturetreatment above 200° C. for two hours, the surface energy reached amaximum value of about 3 J/m² and this value was obtained inconventional silicon direct bonding at temperatures above 1,000° C.

During the experiments the investigators noticed that the nature of thediluted solutions changed after several months and made the bondingnearly impossible. They suggested that atmospheric CO₂ probablyinitiated condensation reactions between the silicate ions, formingsilica sols, and prevented bonding by deposition of small particles onthe wafer surface. The bonding mechanism was proposed to be an initialhydrophilic attraction of two surfaces bring the two wafers into closecontact, and a further condensation reaction in the silicate layersresulting in strong bonds between the two surfaces during the annealingprocess.

Similar studies were also conducted by Yamada et al. See “SOL by WaferBonding With Spin-on Glass as Adhesive” by A. Yamada et al., ElectronicsLetters, 23 (1987) 39, and “Bonding Silicon wafer to Silicon NitrideWith Spin-on glass as Adhesive”, by A. Yamada et al., ElectronicsLetters, 23 (1987) 314. Yamada et al. used commercially availablespin-on glass (SOG) as an adhesive with Si(OH)_(x) as the mainingredient, where 2<x<4, to bond silicon wafers with both silicondioxide and silicon nitride films on the substrates.

The SOG films were spun on the surfaces of two silicon wafers and thewafers were annealed for 10 minutes at 250° C. to facilitate thedehydration process and make the composition of the film SiO₂. theannealed wafers were soaked in organic alkaline solution for 5 minutesand rinsed with water. They were brought into contact with each other ina vacuum and annealed for 1 hour at 250° C. in the vacuum while pressedto each other at pressures less than 10 kg/cm². The bonding was verystrong and the wafers could not be separated without breaking them. Theysuggested that the bonding is due to the enhanced surface reactivity ofthe SOG films which possess a high density of OH groups for dehydrationreactions.

With the foregoing in mind, the subject invention is a method forbonding glass components or other metal oxide forming materials attemperatures below 100° C. The method may be used to fabricate a varietyof devices, including microdevices containing biomolecules or othermaterials that are adversely affected by high temperatures.

The invention includes a method for bonding two mating pieces ofmaterial together using a monomolecular or quasi-monomolecular film thatcovalently bonds to both surfaces. The method is simple, flexible andeffective for low temperature fabrication of microchip devices forcapillary electrophoresis, for example. As such, the method facilitatesthe manufacture, on a commercially viable scale, of microfluidic devicescontaining diagnostic chemical arrays of binding moieties. Applicationsof such devices include medical diagnostics, forensics and genomeresearch. The method also facilitates the inclusion of thin metal filmstructures between two mating substances. Additional applicationsinclude fabrication of optical components that include materials ofdissimilar thermal coeficients of expansion.

EXAMPLE

A standard 50 mm ×25 mm glass microscope slide and 22 mm circularcoverslip were used for bonding experiments. A simple cross microchannelwas generated on the slide using the standard laboratory fabricationprocedures. A sodium silicate solution was prepared from a concentratedcommercial sodium silicate solution (J. T. Baker, SiO₂:28.7%;Na₂O:8.9%).

Diluted solutions with concentrations of 2.0, 5.0, 7.0 and 15 wt. %silicate in water were prepared and used. The substrate with theseparation column and the coverslip were treated as follows:

(1) dip in HF/NH₄F solution and rinse for 1 minute with ultrapure water;

(2) hydrolyze in NH₄OH/H₂O₂ solution for at least 20 minutes, rinse withwater and blow dry with argon gas;

(3) spin-on the diluted solution of sodium silicate in water on thecoverslip at 4,000 rpm for 10 seconds;

(4) immediately bring the coverslip in contact with the glass substratewith the sodium silicate layer in between; and

(5) anneal the sample at 90° C. for 1 hour or room temperatureovernight.

The pH values of the sodium silicate solutions were measured by a pHmeter (Orion, model 290A). The electroosmotic flow of the lowtemperature fabricated microchip was studied using video imaging systemand single point detection setups with Rodamine B (20 μM in 10 mM sodiumtetraborate buffer) as a monitor at a laser wavelength of 514 nm.

Freshly diluted sodium silicate solutions were made from newly purchasedconcentrated solutions and gave strong bonding of the two glass surfacesboth at 90° C. for an hour and at room temperature overnight. A bondingwave was observed once the two glass surfaces were brought into initialcontact, similar to the silicon direct bonding process of Quenzer andBenecke. Within a few seconds the contacting wave spread over the entirearea. The coverslip can be removed by a razor blade after the initialbonding, but it is very difficult to remove the coverslip after theannealing processes.

It is essential to have the two glass surfaces “dry” bonded, i.e.,elimination of all excess water. Any water between the two surfaces willneed higher temperature and longer time to remove, which results in thefailure of the low temperature bonding process. Because of the smallsize and thinness of the coverslip, the sodium silicate solution usuallygets under the coverslip when spun-on its surface, due to theinsufficient initial acceleration of the spinner. This excess amount ofsolution on the edge of the back side of the coverslip will flow inbetween the two bonding surfaces easily during bonding, which may causethe contamination of the separation channel. Therefore, high spin speed(about 4,000 rpm) was used to ensure that both surfaces of the coverslipare dry.

It is desirable to use thicker films to bond to chip surfaces containingstructural features, taking advantage of the gap filling ability of thesodium silicate layer. However, the high spin speed used will give verythin films. This can be solved by using higher solution concentrations.However, when the concentration is too high, the resulting highviscosity of the solution will generate spin-induced radial striations.Also, high pH at high concentration may not give sufficient amount ofSi(OH)₄ group to have good bonding. Although the bond strength was notmeasured, the device made from 15 wt. % solution worked well. The 30 wt.% solution was tried and failed due to the formation of surface radialstriations.

It was found that the shorter the time between finishing the surfacepreparation and pair formation, the easier and more successful thebonding. This may be attributable to the fact that dilution ofconcentrated sodium silicate solution reduces the pH and leads to asupersaturated solution of silicic acid Si(OH)₄ which has highreactivity.

The reactivity of the monomer solution decreases with time due to thepolymerization and colloidal particles formation. The sodium silicatefilm spun-on the coverslip surface will also react with the atmosphericcarbon dioxide and be set by CO₂. See, for example, “Silicate bonding ofInorganic Materials, Part I Chemical Reactions In Sodium Silicate atRoom Temperature” by K. Mackenzie et al., Journal of Materials Science,26 (1991) 769. Thus, the probability of bonding failure increases withthe stand-by time.

For the same reason, the bonding solution has to be freshly prepared,and after the solution has been diluted for a long time, the bondingbecomes very difficult. With old solutions, the two pieces have to bepressed together very hard to get the initial contact wave and there aremany voids formed. Also, the coverslip can be removed by a razor blademore easily. The aging of the concentrated commercial sodium silicatesolution was also observed. Four months after the initial use of thesolution, the bonding became very difficult and much weaker, even thoughthe bonding solution was freshly prepared from the concentratedsolution.

When used as a binder, sodium silicate is known to be set and give greatbonding strength by exposing to CO₂ gas, which is considered to followtwo possible processes: (a) physical dehydration of the sodium silicatesolution by the drying action of the CO₂ gas; and (b) neutralization ofthe silicate and gel formation by chemical reaction with the CO₂. Thus,the aging of the concentrated commercial solution may be attributed tothe absorption of atmospheric CO₂ gas. It is known that colloidalparticles are present together with HSiO₃ ⁻ ions in solutions of sodiumsilicate having SiO₂:Na₂O ratios greater than 2:2. The absorption of CO₂may neutralize part of the alkali and release the silica from the HSiO₃⁻ ions. This causes the growth of the colloidal particles and decreaseof the reactivity of the diluted solution. Since CO₂ is a weak acidicgas, the pH of the bulk solution may not change much when the solutionis aged (pH measured for newly purchased solution—12.5 and for 4 monthold solution—12.4). A bottle of sodium silicate solution (EM Science)was observed to have the silica completely precipitated (pH measured12.6), which can be explained by the fact that when the colloidalparticles are large enough, sedimentation by gravity causes 0precipitation. The main bonding mechanism is believed to be siloxanebond formation between the sodium silicate layer and the glass surfaces.When the concentrated solution is diluted and pH is reduced, theconcentration of hydroxyl ions is no longer sufficient to keep thesilicate ions from being hydrolyzed to Si(OH)₄. Thus, a supersaturatedsolution of silicic acid is formed with high reactivity, which leads tothe condensation-polymerization with any OH-bearing surfaces, formingsiloxane bonds until the supersaturation is relieved. The pH values forthe bonding solutions were measured as follows:

Original concentrated: pH=12.5

15 wt. %: pH=11.9

7 wt. %: pH=11.6

2 wt. %: pH=11.2

Initially, the two hydrophilic surfaces are attracted to each other andthen connected by hydrogen bonding. After that, the siloxane bondformation occurs in the annealing process. the cationic bridging mayalso play a role in the bonding process.

FIG. 5 shows the electroosmotic flow of the chip made by sodium silicatebonding (7 wt. % and at 90° C. for 1 hour). The column was flushed withwater for 20 minutes and then 10 mM sodium tetraborate buffer (pH 9.2)for another 20 minutes before each monitoring of the electroosmoticflow. After the first electroosmotic flow measurement, the chip wastreated with 1 N NaOH for 20 minutes, and the electroosmotic flow wasmeasured again. Another measurement was made after the chip was treatedwith 1 N NaOH for an additional hour. For comparison, a chip made fromthe high temperature direct bonding method was tested in the same way,and the results are shown in FIG. 6.

It can be seen from FIGS. 5 and 6 that there is no significantdifference in the device performance between the chip made from sodiumsilicate bonding and that made from the high temperature direct bonding.As mentioned before, when the solution is aged, the bonding becomesweaker, but it still provides a satisfactory channel sealing. Althoughthe strength of the bonding has not been measured, the device can beoperated under pinched injection and separation at the voltageapplication of 1 kV for at least 50 cycles without any channel leakageobserved.

Since the sodium silicate layer is only on the top side of theseparation channel, it is assumed that the thin layher has turned tosilica after the annealing process and the residual sodium silicate withlower SiO₂:Na₂O ratios can be washed away by water so that it will notaffect the electroosmotic flow.

To check this point, a chip made from sodium silicate bonding was firstflushed with 1 N NaOH for 20 minutes and then tested. Afterwards, it wasfilled with 7 wt. % diluted solution in the channel and dried in 90° C.for an hour before testing. Finally, the chip was again treated with 1 NNaOH for 20 minutes and tested. The results are shown in FIG. 7, whichshows that the sodium silicate solution does not affect theelectroosmotic flow significantly. However, the sample plug becomesnon-uniform with a tail and this may be caused by the residual sodiumsilicate in the channel which changes the charge distribution of thechannel surface. Also, when the channel was flushed with NaOH for longerperiods of time, the electroosmotic flow eventually became slower,differing from the chip made from the direct bonding, as shown both inFIGS. 5 and 7. The NaOH may turn the residual sodium silicate in thechannel with lower SiO₂:Na₂O ratio to that with higher ratio, and thusconsume the OH group on the surface.

In view of the above, the sodium silicate bonding methodology of thepresent invention has proven to be a simple, flexible and effectivemethod for low temperature fabrication of microchip devices forcapillary electrophoresis. Strong bonds have been achieved at roomtemperature and 90° C. with good channel sealing. There is nosignificant difference in the device performance between the chip madefrom sodium silicate bonding and that from the high temperature directbonding. Although when the bonding solution is aged, the bonding becomesweaker, the channel sealing is still satisfactory and the device can beoperated under normal conditions for at least 50 cycles without anyleakage observed.

Although the results noted above relate to microchip fabrication, theprincipals of the invention apply to numerous other analogous uses. Forexample, the bonding technique can be used to bond glass opticalcomponents. The solution can be used as a bonding filler to allowinclusion of structures between glass components (e.g., thin-film metalelectrodes). The methodology can also be used when bonding materials,such as optical components, that have dissimilar thermal coefficients ofexpansion. In addition, the method can also be used to bond other metaloxide forming materials together.

In FIG. 8, a sample composite structure 10 is shown. The structure 10includes a base plate 12 and a cover plate 14. The base plate may bepatterned using photolithographic techniques to form channels 16 inwhich microfluidic manipulations may be made. Both the base plate 12 andthe cover plate 14 are made of glass and can be made from standardmicroscope slides.

The sodium silicate agueous solution is applied to one of the matingsurfaces of the two plates by a known spin-on technique.

The plates 12 and 14 could be any suitable mating components toconstruct other devices, such as optical devices, and may includemetallic connections therebetween or thereon.

With respect to the bonding methodology, an alternative method involvesheating the substrate so that it is at approximately 50° C. at the timeof contacting with the sodium silicate coated coverplate.

The present invention is effective in bonding together two similar ortwo dissimilar materials using a monomolecular or quasi-monomolecularfilm that has the ability to covalently bond to one of the materialswith a reactive moiety at one position on the molecule and covalentlybond with the other material with a reactive moiety at another positionon the molecule.

The invention is particularly suitable for bonding glass materials, andparticularly silica-based materials. The bonding agent is preferablysodium silicate, but other chemically similar materials can be employed.For example, the film could be comprised of an alkoxy silane in analchohol. Thus, while an aqueous solution is preferred for sodiumsilicate, other solutions can be employed for other bonding agents.Also, the bonding agent could include silicic acid and polymeric formsof silicic acid.

With respect to sodium silicate, the solution has a preferred range of0.5 wt. % to less than 30 wt. %.

The materials bonded together can be similar (glass/glass) or dissimilar(meaning either different materials or chemically the same materialshaving different properties). It is, for example, possible to bondplastic to glass, materials having different coefficients of thermalexpansion, etc.

The bonding techniques described herein are also useful in forming avariety of different types of devices, including the following:

Fabrication of In-Column Electrode

A metal electrode can be fabricated in a microchip column using twomasks as shown in FIG. 9. The simple cross-column was generated withmask pattern 1 on a glass slide using standard fabrication procedures.After fabrication of the separation channel, the metal electrode isaligned and put into the injection cross with mask pattern 2 using thetwo alignment marks shown in the upper-left and lower-right regions ofeach slide.

The process for fabricating an electrode is shown sequentially in FIG.10, and can be summarized as follows:

(I) sonicate the substrate with the separation channel in acetone for 5minutes;

(ii) spin on photoresist at 500 rpm for 20 seconds and pattern thephotoresist using mask 2 (soft bake, exposure, develop and hard bake);

(iii) dip the substrate in HF/NH₄F/H₂O etch solution for 30 seconds withan etch rate of about 0.32 μm/min.

(iv) deposit Cr/Au 300/1,800 Å at 50° C.;

(v) lift-off in acetone.

In this way the electrode was embedded in the channel floor with a stepof about 500° Å over the channel surface. the measured resistance of theelectrode is around 50 Ω.

The aforementioned sequence of steps is shown in FIG. 10, where at firstthe substrate 20 is provided with a channel 22. In FIG. 10(b), aphotoresist layer 24 is applied. In FIG. 10(e) a metallic layer 26 isformed. This layer 26, after lift-off, becomes the electrode formed inthe channel. A cover plate 28 is bonded to the substrate using themethods described above, including sodium silicate, to complete themicrochip structure.

FIG. 11 shows the fabricated in-column electrode 26 which extends intothe cross area of a microchannel 30. The width of the electrode is about60 μm, and since the electrode 26 is inserted at an angle of 55° C. intothe injection cross, the total length of the electrode along the channelis around 165 μm.

The electrode can be used in a microchip apparatus, as shown in FIG. 12,to measure the voltage at the cross. Other uses of the electrodefabrication technique include integrated circuit fabrication, as well asother related and analogous uses.

While the preferred embodiment of the present invention has been shownand described, it will be understood that it is intended to cover allmodifications and alternate methods falling within the spirit and scopeof the invention as defined in the appended claims or their equivalents.

What is claimed is:
 1. A composite structure comprising: an impermeablefirst mating piece; an impermeable second mating piece; a filmcomprising a material capable of covalently bonding to the first andsecond mating pieces at temperatures of 90° C. or less, having athickness of at least one molecular layer, and being disposed betweenthe first and second mating pieces; and a channel structure defined byand disposed between the first and second mating pieces.
 2. A compositestructure according to claim 1, wherein the first and second matingpieces are optical components.
 3. A composite structure according toclaim 1, further comprising an electrically conductive layer formed inat least a portion of the channel structure, wherein the channelstructure is a groove located on a surface of the first or second matingpiece, and wherein bonding the second mating piece to the first matingpiece encloses at least a portion of the conductive layer.
 4. Acomposite structure according to claim 1, wherein bonding of the firstand second mating pieces is carried out at room temperature.
 5. Acomposite structure according to claim 1, wherein the first and secondmating pieces are made of materials having different coefficients ofthermal expansion.
 6. A composite structure according to claim 1,wherein each of the mating pieces are made of glass.
 7. A compositestructure according to claim 1, wherein the film is applied via spincoating of an aqueous solution of a bonding agent.
 8. A compositestructure according to claim 7, wherein the bonding agent is silicateand the concentration of silicate in the aqueous solution is less than30 wt. %.
 9. A composite structure according to claim 8, wherein theconcentration of silicate is less than about 15 wt. %.
 10. A compositestructure according to claim 7, wherein the bonding agent is silicate,silane, silicic acid, polymeric forms of silicic acid, alkoxysilanes ortetramethoxysilane.
 11. A composite structure according to claim 1,further comprising biomolecules or diagnostic chemical arrays of bindingmoieties located within at least a portion of the channel structure,wherein the channel structure is a groove located on a surface of thefirst or second mating piece, and wherein bonding the second matingpiece to the first mating piece encloses at least a portion of thebiomolecules or binding moieties.
 12. A composite structure according toclaim 11, wherein the biomolecules or binding moieties comprise nucleicacids, nucleic acid probes, enzymes, antibodies, peptides, or proteins.13. A composite structure according to claim 1, further comprising anelectrode located within at least a portion of the channel structure,wherein the channel structure is a groove located on a surface of thefirst or second mating piece, and wherein bonding the second matingpiece to the first mating piece encloses at least a portion of theelectrode.
 14. A composite structure according to claim 1, wherein thecomposite structure is a device for chemical or biochemical analysis.15. A composite structure prepared by a low temperature bonding method,wherein the composite structure is prepared by a process comprising:providing first and second impermeable mating pieces, the first andsecond mating pieces in combination forming a channel structure;applying an aqueous solution of a bonding agent; eliminating excesswater from the applied aqueous solution to form a thin layer on thefirst mating piece, wherein the thin layer is a film of a least onemolecular layer in thickness which covalently bonds to the first matingpiece with a reactive moiety at one position on the molecule andcovalently bonds to the second mating piece with a reactive moiety atanother position on the molecule; and bringing the second mating piecein contact with the first mating piece with the thin layer in between,wherein the bonding method provides for strong bonds at temperatures of90° C. and less.
 16. A composite structure according to claim 15,wherein the bonding agent is silicate, silane, silicic acid, polymericforms of silicic acid, alkoxysilanes or tetramethoxysilane.
 17. Acomposite structure according to claim 16, wherein the bonding agent issilicate.
 18. A composite structure according to claim 17, wherein thebonding agent in the aqueous solution is silicate and the silicate ispresent in a concentration of less than about 30 wt. %.
 19. A compositestructure according to claim 18, wherein the silicate is present in aconcentration of less than about 15 wt. %.
 20. A device comprisingbiomolecules or diagnostic chemical arrays of binding moieties, whereinthe device has a tri-layer structure, wherein a impermeable first layeris in a mating position with respect to a impermeable third layer,wherein at least one of the first layer or the third layer includes achannel structure and biomolecules or diagnostic chemical arrays ofbinding moieties located within at least a portion of the channelstructure, wherein the channel structure is a groove located on aninterior surface of the first or third layer, and wherein the matedfirst and third layers are bonded by a thin layer of dried silicatedisposed between the first and third layers, wherein the thin layerforms a second layer and wherein the bonding is carried out attemperatures of 90° C. or less.
 21. A device according to claim 20,wherein the dried silicate layer is prepared from an aqueous solution ofsilicate with a concentration of less than about 30 wt. %.
 22. A deviceaccording to claim 21, wherein the concentration is less than about 15wt. %.
 23. A device according to claim 20, wherein the first and thirdlayers are each made of glass.
 24. A device according to claim 20,wherein the first and third layers are made of dissimilar materialshaving different coefficients of thermal expansion.
 25. A deviceaccording to claim 20, wherein the device is prepared by methods whichinvolve no heat treatment above room temperature.
 26. A compositestructure according to claim 20, wherein the biomolecules or bindingmoieties comprise nucleic acids, nucleic acid probes, enzymes,antibodies, peptides, or proteins.
 27. A composite structure comprising:an impermeable first mating piece; an impermeable second mating piece; afilm comprising a material capable of covalently bonding to the firstand second mating pieces at temperatures of 90° C. or less, having athickness of at least one molecular layer, and being disposed betweenthe first and second mating pieces; a channel structure defined by anddisposed between the first and second mating pieces, and an insert atleast partially disposed within the channel structure and selected froman electrically conductive layer, an electrode, biomolecules, ordiagnostic chemical arrays of binding moieties.